The field of the present invention relates to the use of formed polyethylene terephthalate (PET) polarizer film in optical-quality plastic parts.
A variety of polarizing films is known to exist. Conventional polarizing films, however, have not been comprised of PET. This fact is primarily due to PET""s inert properties. The use of PET polarizing films in optical-quality polarized parts, as disclosed in the parent application, is therefore a unique innovation. Moreover, as disclosed in the parent application, such polarized parts can include ophthalmic lenses (semi-finished or finished prescription or non-prescription blanks, lenses, goggles, visors, shields), polarized facemasks or shields, and polarized display devices or windows that require low haze.
Two requirements should be met for all such optical applications: (1) controlled and reproducible curving of the polarizing film to accommodate the contour of the optical parts; and (2) controlled positioning of the polarizer film in the optical-plastic construct.
Much of the formed film manufactured for ophthalmic lenses is laminated, that is, a thin material with the desired optical or mechanical properties (polarization, optical density, color, resistance to breakage, etc.) is sandwiched between two additional layers of plastic for easier handling. These sheets are often joined by adhesives but may be chemically bonded. Lamination techniques with adhesives or by thermal heating are well-known (e.g., Plastics Engineering Handbook, pp. 492ff, The Society of Plastics Industry, Inc. New York, 1960). U.S. Pat. Nos. 5,286,419 and 5,051,309, which are incorporated by reference as if fully set forth herein, discuss heating and forming such laminates of one or more support layers in combination with polarizers.
The problem with lamination is that under stress, the joined layers may delaminate, leaving defects that compromise either the appearance or integrity of the final product. Therefore, techniques have also been developed to form free-standing film into desired shapes.
Free-standing film techniques commonly involve heating the film directly to a softening point so that pressure, vacuum, or a combination of both can be used to force the film into a molded shape. A distinct problem with applying these techniques to common polarizer film (e.g., polyvinyl alcohol (PVA)) is that the high heat needed to soften the film sufficiently to slump into the mold""s shape damages the optical and/or mechanical properties of the film. Accordingly, improvements in polarizer films or forming techniques are desirable.
As disclosed in the parent application, polarized film comprised of PET such as that described in U.S. Pat. No. 5,059,356, which is incorporated by reference as if fully set forth herein, has several advantages over PVA, including affordability, significantly better heat, moisture, and solvent resistance, and good mechanical stability.
The present inventors recognized that an optical-quality plastic part utilizing PET film might offer advantages over an optical construct utilizing a conventional polarizer such as PVA film if the inert PET film could be reliably incorporated into the optical construct. Thus, a suitable method to form the PET polarizer film and properly position the film within the construct is desired.
Polyethylene terephthalate is widely used to form plastic bottles via blow molding. Blow molding, however, is not suitable for forming polarized PET film. In blow molding, melted PET material is extruded as a!tube that is then sealed at one end and expanded into a cooled outer mold shape by forcing air or other gases into the interior of the hot plastic tube. First, an enclosed shape such as this is not suitable for most optical constructs, which normally have open, curved shapes. Secondly, and more importantly, this common technique requires melting the PET, which would negate the polarizing effect of this film.
A similar loss of the polarizing effect of the PET film would result if another common technique, vacuum forming of hot or molten material, was used. In addition, vacuum forming requires vias in the mold to remove entrapped air. Such vias cause unacceptable marks on the polarizer film, resulting in optical distortion on the final parts.
Alternatively, the PET film may be curved or formed by techniques commonly known to those skilled in the art as described in, for example, U.S. Pat. Nos. 5,641,372 and 5,434,707, which disclosures are hereby incorporated by reference as if fully set forth herein. In particular, U.S. Pat. No. 5,641,372 describes the use of vacuum and liquid pressure to force a heated sheet of material against the molding surface, whereas U.S. Pat. No. 5,434,707 describes forming a laminated part under heat and pressures in the range of 250-300 psi.
Such techniques used to form conventional polarizer laminates or free-standing PVA films are not appropriate for forming PET polarizer films. In particular, the PET polarizer film has a thickness of xcx9c100 xcexc. Laminates have a minimum thickness of 0.6 mm, which is a significantly larger mass for heat transfer. In addition, the laminate construction (of protective thermoplastic layers on each side of the polarizer film) means that the polarizer will experience less heat than the outer protective layers. This construction means that the polarizing property of the sandwiched film is less likely to be compromised than in a free-standing PET film. On the other extreme, the free-standing polyvinyl alcohol films are commonly only about 30 xcexc thick, and have much less elasticity than the PET polarizer film. Hence, these thinner, weaker films can be forced out of shape more readily than the PET polarizer film.
Accordingly, an improved method of forming a polarizer film into a curved shape to. accommodate the contour of the typical optical application, and then reliably incorporate the formed film into an optical-quality plastic part without degrading the optical and/or mechanical properties of the film is desired.
The preferred embodiments relate to an optical-quality plastic part having a PET polarizing film formed to a curved shape suitable for the optical application, and to a method of reliably positioning the film relative to surfaces of the optical construct. In addition, the optical performance and cosmetic quality of the formed film is preferably maintained at the high level required for optical-quality constructs such as ophthalmic lenses and displays. Various other embodiments may utilize some but not all of the above elements, or may include additional refinements, while obtaining the benefit of an optical-quality plastic part utilizing PET film.